From an environmental viewpoint, recently, the development of a high-compression and highly supercharged engine is actively performed, and a spark plug having an ignitability which is stable in a high-pressure environment is requested. Moreover, a technique has been known in which, in order to improve the ignitability, the sectional shape of a leading end portion of a ground electrode is made trapezoidal (See Japanese Document No. JP-A-H09-129356; See also Japanese Document No. JP-A-2007-242588).
In the conventional art, in the case where a spark plug is used in a high-pressure environment, however, there arises, for example, a problem in that the temperature of a leading end portion of a ground electrode is raised. Accordingly, in the technique for improving the ignitability of a spark plug which is used in a high-pressure environment, there remains room for improvement.
The invention has been conducted in order to solve the above-discussed problem. It is an object of the invention to improve the ignitability of a spark plug which is used in a high-pressure environment.